Previously the tile coordinate was used to offset a CRTC scanout
coordinate within a larger framebuffer. Since 3.36 we're always
scanning out from (0, 0) as we always have one framebuffer per CRTC; we
instead use the tile coordinate to calculate the coordinate the tile has
in the stage view. Adapt calculation to fulfil this promise instead of
the old one.

This also corrects the tiled custom monitor test case.

!1199